Gauge is a measure of the diameter of wire or jewelry. For example, gauge is used to describe the thickness of piercings. Measures in gauge can be confusing because, counterintuitively, a higher unit of gauge indicates a smaller diameter. So, 14 gauge piercing is thicker than 20 gauge.

An extremely important point that is often forgotten about gauge is that there are at least two different measures of gauge: the British (Standard Wire Gauge, SWG) and American (American Wire Gauge, AWG) measures. Although these measures are both referred to as "gauge", they indicate different actual widths: for example, 14 gauge SWG is .080 inches, while 14 gauge AWG is .0640 inches.

(I've been informed that there is a third measure of gauge, abreviated BWG, but I don't know what it is used for.)

One might think that it is appropriate to use SWG when measuring wire in Britain, and to use AWG when in America. However, this is wrong. At least in the United States, the appropriate gauge system to use depends on the type of metal you are measuring. Convention dictates to use AWG measurements for non-ferrous wire (copper, aluminum), and to use SWG for ferrous wire (steel).

So, getting 14 gauge steel wire and 14 gauge copper wire from the same company will mean you will have two different thicknesses of wire.

Also note that under AWG, there is a mathematical relationship between the gauge number and the thickness; that is, given one, you can derive the other. As far as I know, there is no system dictating SWG, and the correlation between gauge and thickness seems derived from historical practice.

There are three main differences between both rifles and pistols, on the one hand, and shotguns on the other. The first major difference would be that of the construction of the barrels. In shotguns, much like in ancient cannon, the bore in which the projectile travels and eventually leaves the gun is smooth and lacks any sort of rifling. Rifling, or the grooves cut in the inside of a rifle or pistol barrel, is used to give the bullet spin. And by giving a bullet spin, one gives the bullet accuracy. Except for cases in which shotgun barrels are specifically manufactured to shoot slugs and therefore rifled (to be used for deer hunting in certain areas), all shotguns are smoothbores.

Another main difference between said firearms is that of the projectile which they shoot. Rifles and pistols shoot bullets, or solid metal projectiles usually made of lead and more often than not jacketed in some fashion by copper. Bullets come in different calibers and are made larger or smaller, and by proxy heavier or lighter in weight, for their intended target. Except in the case of shotgun slugs as mentioned above, shotguns shoot many small round metal pellets, collectively and singularly called "shot," instead of a single projectile. A musket could be used in the same way: fill the barrel with the required powder charge and proceed to fill the barrel with whatever one could find to stuff in it and off you go. A shotgun is meant to produce a great deal of damage one little bit at a time and to cover a wider area than a single bullet in order to deal this damage.

The third and final difference between the aforementioned firearms is that of how one measures the bore. The bore is the internal diameter of the barrel in which the bullet or shotshell's contents travel from the cartridge to the target. For those in the UK, a shotgun "bore" is the same thing as we in America call the "gauge"; and by gauge, I mean the measurement of the diameter of the internal part of the barrel(s) themselves, excluding the choke. Except in the case of the .410 bore, all shotguns are measured not by their diameter in inches, as are rifles and pistols, but by another measure entirely.

Shotgun gauge is measured by the amount of spherical balls that fit the diameter of the bore which equal one pound in weight. Once done, the resulting number of balls is the resulting gauge. Please recall that I mentioned ancient cannon. The reason for measuring gauge in this way is because in ancient times, cannon, too, was also a smoothbore. In the case of cannon, which also fired spherical shot in a smooth barrel, one measured the size of the cannon by the amount of weight that comprised the cannonball. Thus, a 10 pound cannon shot a 10 pound cannon ball. This 10 pound cannonball was of such a diameter that a relationship could be established: all 10 pound cannonballs could be made and be loaded into a 10 pound cannon, itself which had a certain diameter. So, too, it was with muskets. Obviously not as large as cannon, muskets were also referred to by the weight of the projectiles they shot. Because of their diminutive size compared to cannon, one had to fit a smaller projectile into a smaller tube. Therefore, the old way of measuring what kind of bore was being used was turned into fractions of a pound in weight. Thus, if a musket shot a spherical projectile that was 1/16th of a pound, it was called a 16 bore because it would take 16 such projectiles to equal one pound in weight. And in America, the 16 bore was translated into the 16 gauge.

Since modern day shotguns are the direct descendants of ancient cannon and muskets, it is no wonder that shotguns are measured in this way.

A measure; a standard of measure; an instrument to determine dimensions, distance, or capacity; a standard.

This plate must be a gauge to file your worm and groove to equal breadth by.Moxon.

There is not in our hands any fixed gauge of minds.I. Taylor.

2.

Measure; dimensions; estimate.

The gauge and dimensions of misery, depression, and contempt.Burke.

3. Mach. & Manuf.

Any instrument for ascertaining or regulating the dimensions or forms of things; a templet or template; as, a button maker's gauge.

4. Physics

Any instrument or apparatus for measuring the state of a phenomenon, or for ascertaining its numerical elements at any moment; -- usually applied to some particular instrument; as, a rain gauge; a steam gauge.

Relative positions of two or more vessels with reference to the wind; as, a vessel has the weather gauge of another when on the windward side of it, and the lee gauge when on the lee side of it.

The depth to which a vessel sinks in the water.

Totten.

6.

The distance between the rails of a railway.

⇒ The standard gauge of railroads in most countries is four feet, eight and one half inches. Wide, or broad, gauge, in the United States, is six feet; in England, seven feet, and generally any gauge exceeding standard gauge. Any gauge less than standard gauge is now called narrow gauge. It varies from two feet to three feet six inches.

7. Plastering

The quantity of plaster of Paris used with common plaster to accelerate its setting.

8. Building

That part of a shingle, slate, or tile, which is exposed to the weather, when laid; also, one course of such shingles, slates, or tiles.

Gauge of a carriage, car, etc., the distance between the wheels; -- ordinarily called the track. -- Gauge cock, a stop cock used as a try cock for ascertaining the height of the water level in a steam boiler. -- Gauge concussion Railroads, the jar caused by a car-wheel flange striking the edge of the rail. -- Gauge glass, a glass tube for a water gauge. -- Gauge lathe, an automatic lathe for turning a round object having an irregular profile, as a baluster or chair round, to a templet or gauge. -- Gauge point, the diameter of a cylinder whose altitude is one inch, and contents equal to that of a unit of a given measure; -- a term used in gauging casks, etc. -- Gauge rod, a graduated rod, for measuring the capacity of barrels, casks, etc. -- Gauge saw, a handsaw, with a gauge to regulate the depth of cut. Knight. -- Gauge stuff, a stiff and compact plaster, used in making cornices, moldings, etc., by means of a templet. -- Gauge wheel, a wheel at the forward end of a plow beam, to determine the depth of the furrow. -- Joiner's gauge, an instrument used to strike a line parallel to the straight side of a board, etc. -- Printer's gauge, an instrument to regulate the length of the page. -- Rain gauge, an instrument for measuring the quantity of rain at any given place. -- Salt gauge, or Brine gauge, an instrument or contrivance for indicating the degree of saltness of water from its specific gravity, as in the boilers of ocean steamers. -- Sea gauge, an instrument for finding the depth of the sea. -- Siphon gauge, a glass siphon tube, partly filled with mercury, -- used to indicate pressure, as of steam, or the degree of rarefaction produced in the receiver of an air pump or other vacuum; a manometer. -- Sliding gauge. Mach. (a) A templet or pattern for gauging the commonly accepted dimensions or shape of certain parts in general use, as screws, railway-car axles, etc. (b) A gauge used only for testing other similar gauges, and preserved as a reference, to detect wear of the working gauges. (c) Railroads See Note under Gauge, n., 5. -- Star gauge Ordnance, an instrument for measuring the diameter of the bore of a cannon at any point of its length. -- Steam gauge, an instrument for measuring the pressure of steam, as in a boiler. -- Tide gauge, an instrument for determining the height of the tides. -- Vacuum gauge, a species of barometer for determining the relative elasticities of the vapor in the condenser of a steam engine and the air. -- Water gauge. (a) A contrivance for indicating the height of a water surface, as in a steam boiler; as by a gauge cock or glass. (b) The height of the water in the boiler. -- Wind gauge, an instrument for measuring the force of the wind on any given surface; an anemometer. -- Wire gauge, a gauge for determining the diameter of wire or the thickness of sheet metal; also, a standard of size. See under Wire.